Typically, illumination of a roadside billboard sign is provided, for example, by 2 to 4 high-power metal halide lamps placed in separate fixtures at the base of the billboard sign a few feet out and pointed upward towards the board which may be 15 to 20 feet high and 40 feet or more wide. The lamp fixtures are typically separated from one another by ten to twelve feet. The resulting light distribution on the billboard (irradiance, or power per unit area) is poor and can vary by 6:1 or more, and will often exhibit an undesirable scalloped pattern at the base where, directly in front of the lights, the billboard is most brightly lit and between the lights the billboard is poorly lit. In addition, the irradiance along the height of the billboard is not uniform, decreasing significantly from the bottom to the top of the billboard.
Current billboard lighting systems utilizing a metal halide lamp have a reflector surrounding the backside of the lamp, and a window enclosing the unit. See, for example, U.S. Pat. No. 6,773,135 to Packer, and U.S. Pat. No. 4,954,935 to Hammond et al. Light from the lamp can take two paths before striking the billboard or vertical surface. The first is the direct path from the lamp, through the window to the billboard. The second is the reflected path in which light leaves the lamp, strikes the reflector, exits the window, and then strikes the billboard. In both Packer and Hammond et al. the window consists of a smooth area directly in front of the lamp, and a refractive prismatic-like structure along the periphery of the window. The refractive portion of the window captures some of the direct-path light and bends it toward the billboard. This light would otherwise miss the billboard if it were not refracted and bent by the prismatic structures of the window.
The reflector reflects the reflected-path light from the lamp and distributes it in a controlled fashion across a pre-defined region of the billboard. This may be done by faceting or shaping the reflector. The reflected-path light exits the fixture through the smooth area in the center portion of the window.
The center portion of the window is left smooth so as not to alter the path of the reflected light. But in doing so, the direct light passing through this center portion remains uncontrolled. This presents a problem. In general it is best to control all the light emitted from the lamp, both the direct-path light and reflected-path light to obtain the desired uniformity and light distribution across the billboard or vertical panel. Each point on the window passes both direct light and reflected light. If one tries to control the direct light by manipulating the structure of the center portion of the window, then one adversely affects the path of the reflected light. Conversely, if one tries to alter the path of the reflected light by manipulating the structure of the center portion of the window, then the direct light is adversely affected. Both paths cannot be controlled by the same structure. Although attempts have been made to control the reflected light by structuring the reflector and having a clear window, this allows much of the direct light to strike the billboard uncontrolled or even miss the billboard surface altogether, the consequence of this is that uniformity of the light distribution on the billboard is degraded. It has been found that currently used metal halide lamp systems despite such attempts have poor uniformity of a 6 to 1 variation of the light irradiance across the billboard. Thus, improved lighting apparatuses are needed to overcome the above problem and provide better uniformity of illumination over the entire area of a billboard, or other vertical surface requiring uniform oblique illumination to avoid obstructing the view of such surface from at least the front thereof.
Concerns about efficiency, light pollution, and other factors have manufacturers seeking alternatives to the current high intensity discharge (HID) lamps, such as LEDs. U.S. Pat. No. 7,896,522 to Heller et al. describes a front illuminated billboard using a linear array of LEDs stretching across the entire bottom of a panel to be illuminated. Some of the LEDs are fitted with lenses that are to illuminate a “top” area, others with different lenses to illuminate the “middle” and others that act as “fillers” which may or may not have lenses. The lenses are not designed for the oblique illumination of a billboard or vertical surface. Although useful to improve the overall efficiency for illumination of a billboard from that of HID lamps, the uniformity of this approach is even poorer in that the irradiance along the billboard varies from 12.6 footcandles to 99 footcandles, as stated by Heller et al., or a variation of nearly 8 to 1. It would thus be desirable to provide improved lighting apparatuses that cannot only provide more uniformity of illumination of a billboard, or similar vertical surface, but can utilize LED(s) rather than the typically used HID lamps.